重点是第二问.
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重点是第二问.
![](http://img.wesiedu.com/upload/b/6c/b6c52b330372b61ce9455e7ff5c69399.jpg)
![](http://img.wesiedu.com/upload/b/6c/b6c52b330372b61ce9455e7ff5c69399.jpg)
![重点是第二问.](/uploads/image/z/14885282-2-2.jpg?t=%E9%87%8D%E7%82%B9%E6%98%AF%E7%AC%AC%E4%BA%8C%E9%97%AE.)
a(2)b(2)=12,S(3)+b(2)=3a(2)+b(2)=20
解得a(2)=6,b(2)=2
故d=3,q=2
a(n)=3n,b(n)=2^(n-1)
裂项求和
c(n)=2^(n-1)/[2^(n-1)+1][2^n+1]
=1/[2^(n-1)+1]-1/[2^n+1]
T(n)=1/[2^0+1]-1/[2^1+1]+1/[2^1+1]-1/[2^2+1]+……+1/[2^(n-1)+1]-1/[2^n+1]
=1/[2^0+1]-1/[2^n+1]
=1/2-1/[2^n+1]
再问: 嗯谢谢~
再答: 不客气。
解得a(2)=6,b(2)=2
故d=3,q=2
a(n)=3n,b(n)=2^(n-1)
裂项求和
c(n)=2^(n-1)/[2^(n-1)+1][2^n+1]
=1/[2^(n-1)+1]-1/[2^n+1]
T(n)=1/[2^0+1]-1/[2^1+1]+1/[2^1+1]-1/[2^2+1]+……+1/[2^(n-1)+1]-1/[2^n+1]
=1/[2^0+1]-1/[2^n+1]
=1/2-1/[2^n+1]
再问: 嗯谢谢~
再答: 不客气。